Syrian forces pounded the battered city of Homs on Saturday and blocked aid reaching civilians stranded for weeks without food and fuel in the former rebel stronghold, activists and aid workers said.
The renewed government assault came a day after UN Secretary-General Ban Ki-moon said he had received "grisly reports" that President Bashar al-Assad's troops were executing, imprisoning and torturing people in Syria's third largest city.
"In an act of pure revenge, Assad's army has been firing mortar rounds and 500mm machine guns since this morning at Jobar," said the Syrian Network for Human Rights, referring to a neighbourhood adjacent to Baba Amro, the Homs district from which Free Syrian Army rebels pulled out this week after almost a month of bombardment.
"We have no immediate reports of casualties because of the difficulty of communications," it said in a statement.
Concern was mounting for civilians in freezing conditions in battered Baba Amro, where International Committee of the Red Cross (ICRC) trucks were still blocked from entering.
"The ICRC and Syrian Red Crescent are not yet in Baba Amro today. We are still in negotiations with authorities in order to enter Baba Amro. It is important that we enter today," ICRC spokesman Hicham Hassan told Reuters in Geneva.
A Damascus-based ICRC spokesman said Syrian authorities had given the convoy permission to enter but that government forces on the ground had stopped the trucks because of what they said were unsafe conditions.
"There has been fighting there for at least a month. The situation cannot be good. They will need food, it's cold, they will need blankets. And there are injured there that need to be evacuated immediately," Saleh Dabbakeh told Reuters.
Anti-government activists said they feared troops were keeping out the ICRC to prevent aid workers witnessing a massacre of alleged rebels in Baba Amro which had become a symbol of a year-long uprising against Assad.
UN chief Ban explicitly blamed Damascus for the fate of civilians, in some of his toughtest criticism so far.
"The brutal fighting has trapped civilians in their homes, without food, heat or electricity or medical care; without any chance of evacuating the wounded or burying the dead. People have been reduced to melting snow for drinking water," he said.
"This atrocious assault is all the more appalling for having been waged by the government itself, systematically attacking its own people."
Syria's UN ambassador, Bashar Ja'afari, said Ban's remarks included "extremely virulent rhetoric which confines itself to slandering a government based on reports, opinions or hearsay."
Syrian state news agency SANA reported that a suicide car bomber in the town of Deraa, near the border with Jordan, had killed two people and wounded 20 others.
Residents said seven people had been killed and anti-Assad activists denied the attack was a suicide bombing.
Rami Abdelrahman, head of the Britain-based Syrian Observatory for Human Rights, said anti-Assad fighters had earlier on Saturday killed six soldiers and wounded nine in the town of al-Herak, south of Deraa.
A fighter with the rebels, whose ranks include defectors from Assad's army, said they were planning their next steps after losing Baba Amro.
"Our morale, praise God, is high. We do not say we are sorry when someone is martyred, we all hope for this, and for the fall of the regime," said the rebel fighter who declined to be named.
The rebel withdrawal was seen as a major setback for the armed revolt, that began with largely peaceful protests inspired by the "Arab Spring", but escalated amid a bloody government crackdown.
The Syrian government on Saturday said Baba Amro was now "rid of terrorists".
"The authorities restored security and safety to Baba Amro ... ridding it of members of armed terrorist groups who ran amok in it and committed murder and vandalism, turning the locals' lives into a living hell," SANA said.
Wounded British photographer Paul Conroy, who escaped Homs earlier this week, on Friday said he had witnessed Syrian troops carrying out a massacre in heavily-shelled Baba Amro.
"I've worked in many war zones - I've never seen or been in shelling like this," the Sunday Times photographer told Sky News from a hospital bed in central London.
"I'm an ex-artillery gunner so I can kind of follow the patterns - they are systematically moving through neighbourhoods with munitions that are used for battlefields.
"It's not a war, it's a massacre, an indiscriminate massacre of men, woman and children."
The Syrian government said on Friday it would like to express its "sadness and sorrow" at the death of Conroy's Sunday Times colleague, U.S. journalist Marie Colvin, who was killed in the Homs shelling.
The Paris prosecutor's office said on Friday it had opened a preliminary investigation for murder and attempted murder into the bombing that also killed French photographer Remi Ochlik and seriously wounded journalist Edith Bouvier.
The United Nations says Syrian security forces have killed more than 7,500 civilians since the revolt began last March. Syria's government said in December that "armed terrorists" had killed more than 2,000 soldiers and police during the unrest.